1. Description of the Related Art
In color image forming apparatus, a color image is formed by superimposing images of a plurality of colors. Therefore, if each image is not formed at a proper location, the images can not be superimposed properly, and, this leads to a color image that is degraded.
Some image forming apparatuses see, for example, Japanese Patent Laid-Open Publication Nos. 11-119504 and 2000-155453, have a function to correct the deviations in the positions of the images. Such deviations can occur due to various factors. These factors include machine assembly error, component inaccuracy, changes in environmental temperature, and changes in temperature inside the apparatus.
The image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 11-119504 includes an image-position-displacement detecting unit that detects positional deviation of respective images. An image forming position of one image is corrected through a manual operation, and the image forming positions of the remaining images are automatically corrected based on information from the image-position-displacement detecting unit.
On the other hand, in the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-155453, the correction of the positional deviation is not performed if the image to be formed is a monochrome image. This leads to reduction in the processing time.
Typical schemes for color image formation include a direct transfer scheme and an intermediate transfer scheme. In the direct transfer scheme, toner images of different colors formed on a plurality of photosensitive members are transferred directly onto a transfer paper. On the other hand, in the intermediate transfer scheme, toner images of different colors formed on photosensitive members are first overlaid onto an intermediate transfer member, and are then collectively transferred to the transfer paper. In either schemes, the photosensitive members are aligned facing the transfer paper or the intermediate transfer member. Therefore, such an image forming apparatus is called of tandem type, in which an electro-photographic process, such as forming and developing an electrostatic latent image of each of magenta (M), cyan (C), yellow (Y), and black (K), is performed for each photosensitive member, and then a transfer process is performed on running transfer paper in the direct transfer scheme, and a running intermediate transfer member in the intermediate transfer scheme.
In such a tandem-type color image forming apparatus, an endless belt is generally adopted. In the direct transfer scheme, a recording medium conveyor belt that runs while supporting the transfer paper serves as the endless belt. In the intermediate transfer scheme, an intermediate transfer belt that receives images from the photosensitive members and carries these images serves as the endless belt. Furthermore, in general, an image processing unit including four photosensitive members is placed at one side of the running endless belt. With such structure, in the tandem-type color image forming apparatus, it is important to accurately overlay toner images of different colors one another, to prevent the occurrence of unevenness in color. Therefore, a technique of registration of the respective color images plays an important role. Unevenness in each color is mainly caused by:
skew;
registration displacement in a sub-scanning direction;
scaling error in a main-scanning direction; and
registration displacement in the main-scanning direction.
To place colors in register, various schemes have been devised.
For example, Japanese Patent Laid-Open Publication No. 2001-290327 discloses a technology in which a toner pattern for color matching is formed for each color on an endless belt, a sensor detects a color displacement of a toner pattern of a color with respect to a toner pattern of a reference color, and a color controlling scheme is adopted in which, write timing by an optical writing device is adjusted depending on the detection result.
Here, the toner pattern scheme is described with reference to FIG. 57. FIG. 57 illustrates one example of toner patterns 701 for color matching formed on an endless belt 700, which is a recording-paper conveyor belt or an intermediate transfer belt. The toner patterns 701 include a straight-line pattern orthogonal to the moving direction of the endless belt 700, and an oblique line pattern formed for each of K, Y, C, and M. The toner patterns 701 are detected by sensors 702 to 704 aligned in a main-scanning direction, and measure a skew with respect to a reference color (in this case, black K), a registration displacement in a sub-scanning direction, a registration displacement in a main-scanning direction, and a scaling error in a main-scanning direction. From the measurement results, a main CPU calculates various amounts of displacement and correction for correcting displacement components.
A mirror provided inside the optical writing device reflects a laser beam of each color. A skew displacement is corrected by changing a tilt of the mirror. A stepping motor is used as a driving source for tilting the mirror. Registration displacements in the main-scanning direction and the sub-scanning direction are corrected by adjusting write timing. Furthermore, as a result of mark detection and calculation, when scaling in the main-scanning direction is displaced with respect to the reference color, a device such as a clock generator, capable of changing the frequency in extremely small steps, is used to change the scaling.
Japanese Patent No. 3450402 (hereinafter, (4)) discloses a color image forming apparatus that includes an image forming unit, and an environmental state detecting unit that detects environmental states including temperature and humidity near the image forming unit. When the detection results including the temperature and humidity detected by the environment state detecting unit exceed a predetermined value during continuous formation of a plurality of images, this apparatus causes a pattern image forming unit to form a predetermined pattern image, and then causes a pattern reading unit to read the predetermined pattern image formed, thereby performing registration control.
An image forming process of an image forming apparatus (color image forming apparatus) using the conventional technologies is as follows.
FIG. 53 illustrates the structure of a tandem-type color image forming apparatus in which image forming units are aligned along a conveyor belt. The apparatus is of direct transfer type without using an intermediate transfer member.
Four image forming units that each form an image of a different color (magenta M, cyan C, yellow Y, or black K) are aligned in line along a conveyor belt 802 that conveys transfer paper 801 as recording paper. The conveyor belt 802 is wound between a driving roller 803 for driving rotation, and a driven roller 804 for driven rotation, and is driven by the rotation of the driving roller 803 for rotation in a direction indicated by an arrow. A paper feeding tray 805 is provided below the conveyor belt 802, to accommodate the transfer paper 801. Of the accommodated transfer paper 801, one sheet on top is fed at the time of image formation, and is then attached on the conveyor belt 802 by electrostatic charging.
The attached transfer paper 801 is conveyed to a first image forming unit (magenta), where a magenta image is formed. The first image forming unit (magenta) includes a photosensitive drum 806M, a charging unit 807M disposed near the photosensitive drum 806M, an exposing unit (not shown), a developing unit 809M, and a photosensitive-drum cleaner 810M. The surface of the photosensitive drum 806M is uniformly charged by the charging unit 807M, and is exposed by the exposing unit to laser light 811M corresponding to an image of magenta, to form an electrostatic latent image. The laser light 811M is emitted from a laser light source 805A including laser diodes (LDs) corresponding to magenta, cyan, and yellow.
The electrostatic latent image formed is developed at the developing unit 809M to form a toner image on the photosensitive drum 806M. This toner image is transferred by a transfer unit 812M at a position where the photosensitive drum 806M comes in contact with the transfer paper 801 on the conveyer belt 802 (hereinafter, a transfer position), to form an image of single color (magenta) on the transfer paper 801.
The unnecessary toner remaining on the surface of the photosensitive drum 806M after transfer, is cleaned by the photosensitive-drum cleaner 810M to prepare for the next image formation.
The conveyor belt 802 conveys the transfer paper 801 with single color (magenta) transferred by the first image forming unit (magenta), to the second image forming unit (cyan). In the second image forming unit, a toner image (cyan) formed on a photosensitive drum 806C is transferred in a similar manner described above, and is overlaid on the transfer paper 801. The transfer paper 801 is further conveyed to a third image forming unit (yellow), and then a fourth image forming unit (black), in each of which a toner image similarly formed is transferred, to finally form a multi-color image.
After passing through the fourth image forming unit, the transfer paper 801 with the multi-color image formed thereon comes off from the conveyer belt 802, is fixed by a fixing unit 813, and is then delivered outside the color image forming apparatus.
Detection sensors 814, 815, and 816 are provided below the conveyor belt 802 of the color image forming apparatus body, for detecting a pattern of mark rows for positional displacement detection.
A center shaft of a paper feeding roller 900 is connected to an output shaft of a driving motor (not shown) via an electromagnetic clutch 910 (refer to FIG. 47). The paper feeding roller 900 is described further below. Paired resist rollers 901, one of which has a center shaft connected to the output shaft of the driving motor, are disposed approximately closely attached together. The paper feeding roller 900 and the paired resist rollers 901 separate and feed the transfer paper 801 onto the conveyer belt 802.
FIG. 54 illustrates part of a toner mark row 817 for positional displacement detection formed on the conveyer belt 802 of the color image forming apparatus shown in FIG. 53. FIG. 55 illustrates positional fluctuations by driving of the conveyor belt of the color image forming apparatus in shown FIG. 53. In FIG. 55, the horizontal axis is a time axis, while the vertical axis is a fluctuation amount axis.
Four straight lines of colors K, Y, C, and M (lines orthogonal to a conveying direction of the conveyor belt), and four oblique lines of these colors shown in FIG. 54 are taken as a set of marks, and eight such sets are disposed along the conveying direction to form the mark row 817. The mark row 817 including eight such sets of lines is positioned according to a positional fluctuation phase caused by driving speed fluctuations, such as those caused by the running of the belt in the sub-scanning direction (in a direction represented by an arrow 890). As shown in FIG. 55, the mark row 817 is formed in consideration of the phase, so that an error at the time of pattern formation and detection is as small as possible. By determining a correction amount from an average of the detection results, a high-quality image with less positional displacement in each color can be formed.
With eight straight lines and oblique lines of K, Y, C, and M being formed and then detected by the sensors 814, 815, and 816 aligned in the main-scanning direction, a skew, registration displacement in the sub-scanning direction, registration displacement in the main-scanning direction, and scaling error in the main-scanning direction with respect to the reference color (in this case, BK) can be measured. By shifting the image in a direction reverse to a direction of positional displacement by half a maximum amount of positional displacement detected by each sensor, correction can be performed so that the displacement amount caused by a scaling deviation in the main-scanning direction is made inconspicuous. A main CPU gives instructions for calculating various displacement and correction amounts, and correcting the various values. A cleaning unit cleans the pattern after detection.
Such positional displacement correction is conventionally performed upon instruction from a user menu, a service menu of the apparatus, or upon instruction from a printer driver. Such timing of positional displacement correction is referred to as timing B, while timing of positional displacement correction automatically performed by the color image forming apparatus is referred to as timing A, as disclosed in Japanese Patent Laid-Open Publication No. 2003-186278 (hereinafter, (5)).
However, the image forming apparatuses disclosed in Japanese Patent Laid-Open Publication No. 11-119504 and Japanese Patent Laid-Open Publication No. 2000-155453 discussed above have the following problems.
Consider the case where the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 11-119504 is applied to a system in which a pattern for positional displacement correction is formed on the transfer belt, and is then detected by the sensor, and the displacement amount is measured based on a signal from the sensor and is then fed back to a correcting unit for correction of the positional displacement. In this case, the image concentration of the pattern for positional displacement correction has to be at a sensor-detectable level. The sensor cannot accurately detect the pattern if the pattern is faint, thereby making it impossible to perform positional displacement correction, and leading to degradation in image quality.
Furthermore, while an image positional displacement in color images degrades image quality, an image positional displacement in monochrome images may pose no problem. For example, even if positional displacement correction cannot be made, thereby significantly degrading image quality and requiring checking and repair, it may be desireable to avoid shutting down the apparatus until a service person comes, if such displacement does not affect outputs of monochrome images.
Still further, in the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-155453, image positional displacement is not performed for monochrome image formation. This is because image positional detection does not affect monochrome images irrespective of whether image positional displacement correction is performed. Thus, the processing time at the time of monochrome outputs reduces. There image positional displacement correction is performed only at the time of color outputs. However, if such correction cannot be made, and therefore the apparatus is stopped because image quality is too degraded, neither color images or monochrome images can be output.
Still further, according to Japanese Patent Laid-Open Publication No. 2001-290327, when the number of continuous printed sheets exceeds a predetermined number of sheets for register during continuous printing, a positional displacement correcting process is performed by changing an interval of conveying recording paper, so that the interval is longer than one cycle length of the photosensitive member, while the pattern for positional displacement detection for positional displacement correction is being formed.
However, as described above, when the interval of conveying recording paper is changed while the patterns for positional displacement detection are being formed, a paper conveying control and an electro-photographic process control have to be separately performed in timing, depending on whether the patterns are being formed. This leads to complexity in control, and a positional displacement due to the difference in timing of controls, thereby degrading the image.
Still further, in the conventional technologies described above, an interval between recording paper sheets is minimized as much as possible to increase productivity during continuous printing. Therefore, when positional displacement correction is required, a pattern having a long length, such as the pattern to be formed at the timing B, cannot be formed, and only a pattern having a short length can be used for positional displacement detection. This degrades detection accuracy to cause a positional displacement, thereby degrading the image.